1. Field of the Invention
The present invention concerns cellular mobile radio systems.
The present invention is more particularly, although not exclusively, concerned with cellular mobile radio systems in which access by a mobile station to a cell is effected by means of a particular frequency specific to that cell, called the control frequency, on which synchronization information enabling the mobile station to synchronize to the base transceiver station of that cell is broadcast and on which information enabling a mobile station to access the system in order to set up a call is exchanged.
The control frequencies are generally transmitted continuously to enable the mobile stations listening to them to carry out measurements in order to select the best cell to access at any time.
In systems using the frequency hopping technique (which, among other things, improves transmission quality in the presence of fading), there is generally provision for separate and continuous transmission of the control frequency.
This represents a penalty in terms of spectral efficiency.
2. Description of the Prior Art
Document U.S. Pat. No. 4,723,266 or document WO 93/12589 describes a cellular mobile radio system made up of two types of cell (e.g. microcells and umbrella cells) of which one type is dependent on the other, from the point of view of their radio coverage, and has no control frequency. In this system, access by a mobile station to a cell with no control frequency is effected via a cell with a control frequency, a change of cell (or "handover") then being performed to the cell with no control frequency.
In a system of this kind the problem arises of selecting the best cell to access for a given position of the mobile station, since it is not possible to listen to the control frequencies in the case of cells with no control frequency.
In the documents mentioned above, this problem is solved by providing in each cell, whether communicating with a mobile station or not, specific means for listening to the mobile station enabling the system to determine the best cell by comparison of the various measurement results obtained in this way. This has the disadvantage of requiring, in the cells with no control frequency, dedicated equipment in addition to the equipment needed to handle wanted information.
One object of the present invention is to avoid this drawback.
However, the present invention also applies to cellular mobile radio systems in which all cells have a control frequency.
In such systems, and as described in document FUNKTECHNIK, vol. 41, n.sup.o 4, April 1986, HEIDELBERG DE, pages 146-149, XP002006000, HALAMEK ET AL: "Zellgrenzdetektion mit relativer Entfemungsmessung in Netz C", cell selection can be effected by determining the position of a mobile station by measuring the propagation time difference between the mobile station and two base transceiver stations. To be more precise, a base transceiver station BSa with which the mobile station is communicating measures the propagation time Ta between that base transceiver station and the mobile station and transmits that propagation time to the mobile station which then retransmits it, in which case the propagation time is received by another base transceiver station BSb (which can then, assuming that the system is synchronized), i.e. that the base transceiver stations are synchronized, determine the propagation time Tb between the mobile S station and the base transceiver station BSb and thus determine the difference between the propagations Ta and Tb. One disadvantage of a solution of this kind is therefore that it works only in the case of a synchronized system.
One object of the present invention is to avoid this drawback.